Hi Oscar.
A sunspot is caused by areas of strong magnetic field that come up from below the surface of the Sun. The larger the area of strong magnetic field the larger the sunspot.
There is more magnetic field coming to the surface like this at times of lots of solar activity (solar maximum), so often you get your largest sunspots then rather than around times of low solar activity (like now - we are very close to the minimum of the 11 year solar cycle)
None of this explains why one sunspot region is large while another on the sun at the same time is small. We are still trying to figure that out!
You can read more about sunspots here:
http://www.windows.ucar.edu/tour/link=/sun/atmosphere/sunspots.htmlcheers,
Terry